The present invention relates to digital rounding techniques, and more particularly to an adaptive rounder for video signals that introduces a dither signal only when there is a rounding error, the dither signal being a fixed grid for "freeze frame" inputs but random during changing inputs, and being filtered to remove components at the subcarrier frequency for composite input video signals.
Digital processing of video signals often results in an increase in dynamic range, i.e., an increase in the number of bits representing a video sample. This increased number of bits usually is reduced at some point to control costs and/or to meet digital interconnection standards. Straight truncation or rounding of the digital signal at the output of a video processing module often introduces objectionable correlated errors. For example, rounding off signals such as flat fields or ramps may produce a low frequency correlated error to which the eye is particularly sensitive. The introduction of a dither signal randomizes the correlated error and reduces its effect. Although the magnitude of the error in the dithered output is greater as compared to simple rounding, the randomness of the error results in a perceived improvement.
But there are still problems with this method. First the addition of the dither signal reduces the signal to noise ratio of the digital signal all of the time, even when conditions are such that there is no correlated error and the dither signal is really not needed, i.e., when the processing module is not processing and only passing through the signal. Second the dither signal may have components at the subcarrier frequency of a composite video signal that introduces objectionable chrominance noise, which noise is particularly noticeable when processing monochrome video with no chrominance content. Finally dithering a signal that originates in a fixed source and is subject to a fixed processing operation, such as a static frame buffer input to a fixed gain stage, results in temporal noise, i.e., pixels that were constant in time are now changing from frame to frame as a result of the dither. This problem is somewhat subjective, but it is desirable that a freeze frame input to a fixed processing operation remain "frozen". The use of a fixed dither grid, random within a frame but the same from frame to frame, solves this problem, but such a fixed dither grid accumulates linearly in successive processing operations, which is also undesirable.
FIG. 1 illustrates a prior art basic dithering technique without rounding that is disclosed by N. S. Jayant and Peter Noll in "Digital Coding of Waveforms" published by Prentice-Hall, Inc. in 1984. An input signal x(n) is input to an adder together with a dither signal from a dither source, and the output is then quantized to produce an output signal y(n).
One method for rounding down the number of bits in a digital video signal to reduce the visibility of quantization errors over simple truncation is an error feedback technique as described in an article by M. G. Croll, et al of the British Broadcasting Corporation entitled "Accommodating the Residue of Processed or Computed Digital Video Signals within the 8 Bit CCIR Recommendation 601." The input video signal of m-bits is input to an adder together with the (m-t) least significant bits of the output of the adder. The t most significant bits are output to other video devices, with the lowest order bit being affected by the carry from the addition of the accumulated least significant bits of the input signal.
Another method is disclosed in European Patent Application Publication No. 0,270,259 A2 assigned to Quantel Limited entitled "Improvements Relating to Video Signal Processing Systems." To reduce an m-bit signal to an t-bit signal, the least significant (m-t) bits are input to a comparator together with the output of a random number generator. If the (m-t) bits are greater than the random number generated, then the most significant bits are incremented by one. Otherwise the most significant bits are not altered.
What is desired is an adaptive rounder that applies dither only when required, applies a fixed grid dither when the input signal is frozen and a random dither when the input signal is changing, and does not introduce objectionable chrominance noise when the input signal is a composite video signal.